The Bible frequently refers to human nature as "flesh". It is impossible for those who are part of "the flesh" to know, obey, or love God. The sinful nature of humans makes this impossible. To be reconciled to God, a person must undergo a change of nature. There is no way for this to be accomplished by normal means. A person must be born again through the Spirit of God.
All things associated with the flesh are fleshly. The flesh is a symbol of the human nature, the human character, and the human senses. Because we were born in flesh and blood, we are unable to enter God's perfect kingdom, which is why sinful man needs to be reborn. In the absence of spiritual birth, a person cannot see heaven or enter the heavenly portal.
The moment we are saved by grace through faith, the moment we have been born again of the Spirit, we become spiritual beings. Until we are united with Jesus, we live on earth in our fallen bodies, awaiting the day when we will be raised. On that day, this mortal body will be transformed into immortality, which will replace this perishable form with an imperishable one.

Dear Heavenly Father, thank You for providing such a clear and detailed explanation of the good news of the Gospel in Scripture. Thank You for giving us a bird's-eye view of Nicodemus' teaching on the new birth. It is through faith in Christ that I have the opportunity to be reborn into this world and become a child of God. By faith in Christ, I have been declared righteous. Thank You for assuring me that not only will I see heaven, but I will also be welcomed into Your heavenly kingdom.
In Jesus's name I pray. Amen.